1. Field of the Invention
The present invention relates to gray scale displaying capable of a display according to a plurality of luminance data and more particularly relates to an apparatus and a method for a gray scale display capable of displaying color information in accordance with luminance data corresponding to the color information.
2. Description of the Prior Art
Recently, miniaturization of office automation equipment has been extensively developed, keeping pace with the advance of the large scale integrated circuit technology and portable personal computers and the like and steadily extending their market. The chief factor in development of portable equipment is improvements in the display apparatus. Flat display devices such as liquid crystal, plasma, and EL (electro-luminescent) displays superseding the conventional CRT (cathode ray tube) are becoming important. The technology for gray scale display for emulating color display information with a monochromatic display, in particular, is useful in the sense that existing software can be used. As disclosed, for example, in Japanese Laid-open Patent Publication No. 58-57192, the liquid crystal display (hereinafter to be briefly referred to as "LCD") is enabled to make a half-tone display by being controlled such that displaying of statuses and non-displaying of statuses therein are switched at intervals of a certain number of frame cycles. Below wil be described such a gray scale display with reference to FIG. 2.
FIG. 2A is a block diagram of a display circuit in a personal computer showing a prior art example. Referring to the figure, reference numeral 1 denotes a central processing unit (hereinafter to be briefly referred to as "MPU"), 2 denotes an address bus, 3 denotes a data bus, 4 denotes an LCD timing controller (hereinafter to be briefly referred to as "LCTC"), 5 denotes a frame line marker signal (hereinafter to be briefly referred to as "FLM signal"), 6 denotes a selector, 7 denotes a compound bus, and 8-10 denote display memories R, G, and B, respectively. Further, 11-13 denote parallel-to-serial converters (each block thereof in the drawing is briefly labeled "P.fwdarw.S"), 14-16 denote display signal lines R, G, and B, respectively, 17 denotes a gray scale signal generator, 18 denotes a gray scale controller, 19 denotes a video signal line, 20 denotes a display data decoder, and 21 denotes a liquid crystal display (hereinafter to be briefly referred to as "LCD"). Operation of the circuit of FIG. 2 will be described below.
When the MPU 1 accesses the display memories 8-10, the selector 6 selects the address bus 2. Thereby, the MPU 1 is enabled to update or read the contents of the display memories 8-10. When the contents of the display memories 8-10 are to be displayed on the LCD 21, the selector 6 selects display address information which is output from the LCTC 4. Accordingly, the contents of the display memories 8-10 are read out in the sequence of scanning according to the information on the compound bus 7 and these are delivered to their respective parallel-to-serial converters 11-13. The information is thereby converted into R, G, and B display signals on lines 14-16, respectively, and are supplied to the gray scale controller 18 as display information. Meanwhile, the gray scale signal generator 17 delivers eight kinds of gray scale signals (Y0-Y7) to the gray scale controller 18. Of these gray scale signals, Y0, for example, provides the darkest display and Y7 provides the brightest display. The gray scale controller 18 selects a gray scale signal corresponding to the display information for eight colors conveyed by the display signals 14-16 and outputs it to the video signal line 19. Thereby, the circuit is enabled to emulate the eight-color display with a monochromatic display in eight gray scales. The correspondence between colors and gray scales are shown in FIG. 2B. The video signal on line 19, including such gray scale display information, is converted by the display decoder 20 into information that can be displayed on the LCD 21. The LCD 21 makes a display according to the delivered information, with frame synchronization taken from the FLM signal 5. What has just been described is the outline of the operation of the display circuit enabling the LCD to make a display of eight gray scales.
Below will be described detailed operations of the gray scale signal generator 17 and the gray scale controller 18 with which the present invention is most concerned.
FIG. 3 is a timing chart of the gray scale signals Y0-Y7 generated by the gray scale signal generator 17. The reference signal is the FLM signal on line 5 at one frame cycle (approximately 70 Hz). Seven frame cycles are used as a gray scale period. For example, the display for the signal Y1 is turned ON at the rate of 1/7. That is, the display is turned ON during one scan out of seven scans of the picture and it is turned OFF during the period of the remaining six scans. Thereby, the duty ratio of picture displaying is changed and a half tone display can be achieved. The signals Y2-Y6 are the signals that also change the duty ratio so that half tone display may be achieved similarly.
Details of the operation of the gray scale controller 18 will be described below with reference to FIG. 4.
FIG. 4 is a block diagram showing the gray scale controller 18. In the figure, circuit blocks and signal lines corresponding to those in FIG. 2 are denoted by corresponding reference numerals. Referring to FIG. 4, reference numeral 22 denotes a decoder of a three-bit structure, 23-30 denote AND circuits, and 31 denotes an OR circuit. The decoder 22 decodes the color information conveyed by the R, G, B display signal lines 14-16 and turns ON only the AND circuit corresponding to that color. For example, if the color information is that for the black color, the AND circuit 23 is turned ON, or if it is for the blue color, the AND circuit 24 is turned ON. Therefore, the OR circuit 31 outputs the gray scale signal corresponding to the color information to the video signal line 19. The description made so far is of the operation of the gray scale controller 18 converting color information into gray scale information.
As described above, a gray scale display to achieve color emulation can be performed by a monochromatic display apparatus if the color display circuit is additionally provided with the gray scale signal generator 17 and gray scale controller 18. Thus, implementation of portable office automation equipment can be advanced taking over the great assets of application software intended for color display.
In the above described prior art, the correspondence between the color information and the luminance information was fixed. Therefore, there was a problem, when gray scale displaying was conducted using some existing application software intended to produce color information in which characters in "blue" color were frequently used with the background in "black" color, the difference in the luminance between the color of the characters and the color of the background became so small that the characters were difficult to detect. Further, the relationship between the duty ratio dependent on the gray scale cycle and the luminance differs with the characteristics of the LCD used. More particularly, the gray scale signals shown in FIG. 3 may be suitable for LCDs manufactured by a first manufacturer but gray scales, for example, of Y0-Y3 may be difficult to discriminate in LCDs manufactured by a second manufacturer because of uneven differences in luminance between the adjoining gray scales. Therefore, when using LCDs manufactured by the second manufacturer, it becomes necessary to change the timing of gray scale cycles for the gray scale signals (Y0-Y7) and also to change the circuit in the gray scale signal generator accordingly so that suitable luminance differences may be obtained, and this was a difficulty in effective product development.